Recently, as portable electronic instruments are required to have multifunctionality and long service time, it is necessary for lithium secondary batteries used as power sources for such portable electronic instruments to be provided with high capacity.
A battery, for example, a secondary battery uses a metallic can as a battery casing. A part of primary batteries, including lithium batteries, and lithium polymer secondary batteries use a multilayer pouch, which comprises aluminum foil and various types of polymer films, as a battery casing.
In order to impart high capacity to a lithium secondary battery, it is possible to use an anode material and a cathode material having high capacity. In addition, thinning of a battery casing may be useful for providing a battery with high capacity. This is due to reduction in the thickness of a battery casing, such as a can, which enables a surplus amount of electrode active materials to be applied to a battery. Therefore, it is possible to provide a battery with high capacity and high-rate charge/discharge characteristics. However, a can shows poor strength when it is made thinner. Therefore, when the internal pressure of a battery increases due to the gas generated during charge/discharge cycles, or when a jelly roll-like structure expands due to repeated charge/discharge cycles, the can may be swelled, thereby adversely affecting the quality of a battery. Additionally, there is a possibility of a battery pack itself or electric and electronic instruments using the battery pack being damaged during repeated charge/discharge cycles after the assemblage of a battery.
Meanwhile, when a multilayer pouch is used as a battery casing, there is an advantage in that a battery using the multilayer pouch has a significantly decreased weight compared to the corresponding battery using the aforementioned metallic can.
As shown in FIGS. 2 and 3, a lithium ion polymer battery 1, also referred to as a “pouch battery”, comprises: a casing main body 2 having an inner space 2a with a predetermined size; a cover 3 coupled rotatably to the casing main body 2; a predetermined number of unit cells 4, which comprises a cathode 4a, an anode 4b and a separator 4c and is inserted into the inner space 2a of the casing main body 2; connections 5 extending longitudinally and externally from each end of the cathode 4a and the anode 4b in each unit cell; and a cathode terminal 6 and an anode terminal 7, each connected to the corresponding connection.
Herein, an extended portion 2b with a predetermined width is formed at the top edge of the inner space 2a of the casing main body 2, toward the exterior on the horizontal surface, for the purpose of heat fusion. Also, an insulation tape 8 formed of a non-conductive material is coated on the central portion of each of the cathode terminal 6 and the anode terminal 7, connected to the corresponding connection 5. The insulation tape 8 is provided in order to prevent a short circuit between a heat fusion device and an electrode terminal 6 or 7 when the heat fusion device (not shown) performs heat fusion between the extended portion 2b of the casing main body 2 and the edge portion 3a of the cover 3, and to increase sealability between the sealed portions 2b and 3a. 
Then, a predetermined number of unit cells 4, each unit cell comprising the cathode 4a, the anode 4b and the separator 4c, is inserted into the inner space 2a of the casing main body 2. Next, a predetermined amount of electrolyte is injected into the inner space, and then the cover 3 is adhered closely to the casing main body 2, so that the extended portion 2b of the casing main body can be sealed with the edge portion 3a of the cover 3 by using a heat fusion device (not shown) to prevent the electrolyte from leaking.
More particularly, the connection 5 of each unit cell 4 is connected to the corresponding electrode terminal 6 or 7, whose central portion is coated with an insulation tape 8. Additionally, each electrode terminal and a part of the insulation tape 8 protrude out from the casing main body 2 and the cover 3.
For example, each of the casing main body 2 and the cover 3 is comprised of an outer coating layer 9a formed of an oriented nylon film (Ony), a barrier layer 9b formed of Al, and an inner sealant layer 9c formed of a casting polypropylene film (CPP). Additionally, a hot melt layer (not shown) is coated on the edges of the inner sealant layer, so that the extended portion 2b of the casing main body 2 and the edge portion 3a of the cover 3 can be in close contact with each other and fixed to each other by heat and pressure provided by a heat fusion device.
However, in the aforementioned pouch battery according to the prior art, both the casing main body and the cover are comprised of an outer coating layer formed of an oriented nylon film (Ony), a barrier layer formed of Al, and an inner sealant layer formed of a casting polypropylene film (CPP). Hence, when the battery is subjected to a physical impact or is compressed by a sharp object, the casing main body and the cover of the pouch battery is damaged with ease, resulting in safety-related problems, such as infiltration, ignition or explosion. Additionally, there is another problem of poor processability during the assemblage of a battery, caused by the softness of the pouch.